The present invention relates to a method of determining the height position of an electrode which moves vertically in an arc furnace, using to this end an elongated flexible device, preferably a heavy chain. The invention also relates to an arrangement for carrying out the method.
Arc furnaces normally include one or three electrodes. Three electrodes are used in three-phase furnaces with one electrode connected to each phase, whereas only one electrode is used in direct current furnaces. The electrodes are provided with control means which regulate the height of the electrodes so as to maintain an optimal arc between electrode and scrap or between electrode and the molten bath when the scrap reaches this state. A normal working cycle can thus be divided into three phases.
In the first phase, the furnace is filled with scrap and the electrodes "burrow" down through the scrap charge while melting the scrap. In the second phase, the scrap surrounding the electrode collapses into the "burrow" and the electrodes must again be lifted out of contact with the scrap in order to maintain the arcs. The third phase involves stabilizing the electrodes on a lower level above the molten bath in the furnace. The electrodes are constantly moved up and down to maintain the arcs, particularly during the first two phases.
The optimal process control of arc furnaces requires knowledge of the furnace wall temperature and also of the vertical movement of the electrodes therein. A good estimation of the melting process for optimal control thereof can be obtained by studying the temperature course together with movements of the electrode. This enables productivity to be increased and the consumption of energy to be reduced at the same time. Oxygen consumption can also be optimized thereby.
Continuous sensing and recording of the temperature of the furnace wall elements presents no technical problem. On the other hand, difficulties have been encountered in practice in providing a simple, inexpensive and reliable method for sensing and indicating the height positions of the electrodes as a function of time.
The electrodes are mounted on electrode arms to which so-called electrode masts are connected. Attempts have earlier been made to record the movements of the electrode masts with the aid of lines and associated pulleys and drums for driving angle indicators or like devices; see for instance DE-A-2001476 and DE-C2-2651544 in this regard. The environment that surrounds furnaces of this kind is, however, extremely harsh and problematic and consequently all movable elements will cease to function in the passage of time, and the lines used will often break. Unduly thick lines cannot be used, since they would require the use of pulleys and drums of larger diameters.
The use of optical measuring devices has also been proposed. Such devices, however, increase costs and cause problems as a result coatings that form on the optical elements in the passage of time.
The main object of the present invention is to provide a method and an arrangement which will enable the height position of an electrode in an arc furnace to be determined cheaply, reliably and continuously without the use of rotating elements or optical elements. AU-B-87724/82 describes an arrangement for determining the height position of an elevator or hoist cage. This arrangement includes an elongated flexible element which hangs down from the movable elevator cage and the position of said cage is determined at different points in time, by weighing that part of the flexible element that is carried by the cage.
An arrangement of this kind cannot be used in arc furnaces, because, among other things, the weight sensing load cell would be located close to the furnace in an environment which would cause the function of the cell to become highly unreliable. Furthermore, it would be necessary to install electronic cables up to the furnace, which would result in a number of serious difficulties and increase costs.
Furthermore, the known solution, especially when the flexible element has the form of a heavy chain, would cause the load cell to be subjected to dynamic stresses in conjunction with the rapid accelerations of the electrode that the electronic control arrangement can command, especially when the load cell functions to sense the load continuously. In addition to generating mechanical stresses and strains in the load cell, this would also result in erroneous cell signals because the desired electrode position signals would be obscured by signals resulting from these dynamic forces.